Running in the Dark Sector

TL;DR

This paper explores how the running of the dark photon coupling and kinetic mixing with momentum transfer can reveal dark sector properties and impact experimental predictions, especially for strong couplings.

Contribution

It demonstrates the significance of the running of the dark photon coupling and kinetic mixing in dark sector models, providing a new way to probe dark particles through experiments.

Findings

Running of $eta_d$ can be significant and measurable.

Running of $ ilde{ ext{mix}}$ affects experimental predictions.

Models suggest an infrared upper bound on $ ext{alpha}_d$.

Abstract

The "dark photon" $\gamma_d$ of a gauged $U(1)_d$ can become practically invisible if it primarily decays into light states from a dark sector. We point out that, in such scenarios, the running of the $U(1)_d$ "fine structure constant" $\alpha_d$, with momentum transfer $q^2$, can be significant and potentially measurable. The $\gamma_d$ kinetic mixing parameter $\varepsilon^2$ is also expected to run with $q^2$, through its dependence on $\alpha_d$. We show how the combined running of $\varepsilon^2 \alpha_d$ may provide a probe of the spectrum of dark particles and, for $\alpha_d\gtrsim {\rm few}\times 0.1$, substantially modify predictions for "beam dump" or other intense source experiments. These features are demonstrated in simple models that contain light dark matter and a scalar that breaks $U(1)_d$. We also discuss theoretic considerations, regarding the $U(1)_d$ model in the ultraviolet regime, that may suggest the infrared upper bound $\alpha_d \lesssim 0.1$.